It is often desirable to pass a digital data stream across a galvanic isolation barrier. For example, the upper switch of an IGBT half-bridge may require a gate signal that cannot be ground-related, but may need to be driven at a common-mode potential of one or more kilo-volts. Numerous and varied devices such as optocouplers and transformers have been applied to accomplish this task. A modern example of such a digital isolator is the Analog Devices ADuM1100.
It is also often desirable to pass a digital data stream bidirectionally across a galvanic isolation barrier. U.S. Pat. No. 7,773,733 exemplifies such bidirectional transmission. This patent teaches the use of a single transformer to pass half-duplex bidirectional digital signals, i.e., the signals may pass in only one direction at a time.
Sometimes full-duplex operation is needed, for example telephone signals are almost always simultaneously bidirectionally transmitted. For this purpose, so-called hybrid or bridge coils have been used on telephone lines for over a century. In a typical telephone application, each telephone comprises such a coil, each of which usually has three or more windings. Thus the entire connection requires two hybrid coils, one at each end of the connection. Similar means have been used for full-duplex digital transmission, as is taught in U.S. Pat. No. 3,936,602. Such complex magnetics have not proved advantageous for modern digital full-duplex transmission. The Analog Devices ADuM1201 digital isolator is a modern device providing full-duplex digital isolation, employing two transformers, one for each direction of transmission.
Since an IGBT may dissipate excess power if not fully saturated, a desaturation fault signal is commonly passed across the galvanic isolation barrier in the opposite direction from the signal operating its gate. Using half-duplex operation for this transmission incurs complexities to avoid collision of gate data signals and fault signals. To minimize both galvanic barrier capacitance and cost, it is desirable to attain full-duplex transmission with a single, simple transformer.